The architecture of the Long Term Evolution (LTE) system for cellular radio communications uses an X2 logical interface between base stations called eNBs and an S1 logical interface between eNBs and a Mobility Management Entity (MME)/Serving Gateway (S-GW) (S1) as shown in FIG. 1. LTE is based on a “flat” architecture as compared to 2G and 3G systems. Each cell is served by an eNodeB or eNB base station, and handovers between cells can be handled either via a MME and the S1 interface or directly between the eNBs via the X2 interface. The radio access network is referred to as an Evolved-Universal Terrestrial Radio Access Network (E-UTRAN).
A cellular network typically includes some areas with high traffic, e.g., a high concentration of users. In those areas it may be desirable to deploy additional capacity to ensure user satisfaction. The added capacity may be in the form of additional macro base stations (e.g., more eNBs) and/or in the form of lower output power base stations. The latter cover a smaller area in order to concentrate the capacity boost in a smaller area. Examples include micro, pico, home base stations, relays, etc. Often, there are also areas with poor coverage where there is a need for coverage expansion, and one way to address these coverage issues is to deploy a low output power node to provide the coverage boost in a small area. A benefit with lower output power nodes in these situations is that their impact on the macro network is reduced, e.g., a smaller area in the macro network experiences interference.
A network deployment that uses both higher power macro nodes and lower power smaller nodes is referred to here as a heterogeneous network or “HetNet.”Multiple layers in a HetNet are illustrated in the example of FIG. 2. A higher power macro base station (the high tower) provides a wide area coverage called a macro cell, and lower power nodes (the shorter structures) provide small area capacity/coverage in smaller cells. In this example, pico base stations and pico cells, relays and relay cells, and home base stations sometimes called femto base stations and femto cells are shown. Although FIG. 2 shows clusters of femto cells, single femto cell deployments may also be used.
Because cells in a HetNet typically operate with different pilot power levels, there can be imbalances between the radio uplink (UL) and the radio downlink (DL) in the network. Cells are typically selected by user equipments (UEs) based on their measurements of the received signal strength of downlink transmissions from those cells, with UEs being served by the best downlink cell alternative. However, the uplink quality depends mainly on the distance between the UE and the serving base station site and is generally independent of the serving cell's downlink pilot power. As a result of a UE's serving cell/base station selection being based on downlink pilot signals, UEs may have a better uplink signal quality to a non-serving cell. In this situation, Cell Range Expansion (CRE) may be used.
CRE is now described in conjunction with FIG. 3. A Macro UE (MUE) 11 served by the Macro eNB 10 is configured by the Macro eNB to detect further away cells that normally would not otherwise be detected by the MUE. For example, these further away cells may include those with a pilot signal 6 dB lower than the pilot signal of the macro cell 12. The extended area within which the MUE 11 can detect small cells with pilot signals below a predetermined threshold, whose particular value depends on the application, is called the Cell Range Expansion (CRE) area 18 of the smaller cell 16 to create a larger pico cell 20 served by the Pico eNB 14. In order to detect neighbor cells transmitting pilots with lower signal strength, the MUE is configured by the Macro eNB with a CRE measurement offset, as shown in FIG. 3.
Once such smaller cells are detected by the MUE and reported to the Macro eNB, the Macro eNB can decide to handover the MUE's connection to the detected smaller cell, which in FIG. 3 is served by a Pico eNB. The smaller cell and smaller eNB are in non-limiting examples referred to as a Pico cell 16, 20 and a Pico eNB 14, respectively, for illustration purposes only.
In the case of a handover from a macro cell to a Pico cell's Cell Range Expansion (CRE) area by configuring the UE to trigger to report a handover candidate via a larger measurement Offset, as shown in FIG. 3, the UE can report a handover target cell that would not have been a good handover candidate if evaluated without measurement offset. Once the UE is handed over to the CRE area of the Pico cell, it is undesirable for the UE to be immediately handed back over to the macro cell. Such handing back over undermines the benefits of CRE. To prevent this, the inventors realized that there needs to be some type of communication between the macro eNB and the pico eNB to ensure that the UE measurement configuration and any handover decision while camping on a CRE Pico cell are appropriately controlled and coordinated.
Such handover from macro cell to pico cell might be preceded by allocation of so-called Almost Blank Subframes (ABS) by the Macro eNB (see, e.g., 3GPP TS 36.331 and TS 36.423 incorporated herein by reference). ABSs are “protected subframes” where the Macro eNB temporarily limits the output power of its transmission so that UEs served by a smaller cell neighboring the Macro eNB experience reduced interference on such ABS subframes.
After the MUE is handed over to the small cell due to CRE, the small cell eNB may decide to serve the UE on ABSs, due to the otherwise high DL interference the UE would experience from the Macro eNB. Furthermore, the UE may be configured by the small cell eNB so to measure neighboring cells on ABSs to ensure that the measurements are not impacted by high levels of Macro eNB DL interference.
Recently a new technique has been introduced in 3GPP, similar to ABS, called Reduced Power SubFrames (RPSF) which includes subframes where the Macro eNB schedules data traffic for MUEs at a reduced Tx power. The RPSF technique differs from ABS in that no data traffic is supposed to be transmitted on ABS subframes. The RPSF can also be seen as an extension to ABS, where ABS can be said to either be configured with zero power or reduced power data and control transmissions.
To date, the CRE, ABS, and RPSF techniques exist in isolation in that there is currently no provision to coordinate the use of these techniques together or across different eNBs. Given that it is advantageous for a UE connected to a Pico cell and in the CRE area to be served on “protected” subframes, the absence of such mechanisms is a significant problem that needs addressing. Moreover, there is a need to communicate the configuration and use of RPSF between neighboring eNBs, e.g., to help interfered eNBs schedule transmission times for their UEs on protected subframes or to activate one or more specific interference cancellation techniques at the UE if it is known that interfering data traffic will be present on protected RPSF resources.